Brine concentration

ABSTRACT

A process for separating solvent from a feed solution, said process comprising: contacting a feed solution comprising solutes dissolved in a solvent with one side of a nanofiltration membrane, applying hydraulic pressure to the feed solution, such that solvent and some of the dissolved salts from the feed solution flow through the nanofiltration membrane to provide a permeate solution on the permeate-side of the nanofiltration membrane and a concentrated solution on the retentate-side of the nanofiltration membrane; contacting the permeate solution from the nanofiltration membrane with one side of a reverse osmosis membrane and applying hydraulic pressure to the permeate solution, such that solvent from the permeate solution flows through the reverse osmosis membrane to leave a concentrated solution on the retentate-side of the reverse osmosis membrane, using the concentrated solution from the retentate-side of the reverse osmosis membrane as at least part of the feed solution tothe nanofiltration membrane;withdrawing at least a portion of the concentrated solution from the retentate-sideof the nanofiltration membrane.

BACKGROUND

The present invention relates to a process for separating a solvent, forexample, water from a feed solution. In particular but not exclusively,the present invention relates to a process for the purification ofwater.

Various methods of water purification and concentration are known. Anexample of such a method is reverse osmosis. In reverse osmosis, wateris forced from a region of high solute concentration through asemipermeable membrane to a region of low solute concentration byapplying a pressure in excess of the osmotic pressure of the high soluteconcentration solution. Reverse osmosis is commonly used, for example,to obtain drinking water from seawater. Reverse osmosis is also used toseparate water from, for example, industrial waste streams. By usingreverse osmosis to treat industrial waste streams, it is possible togenerate relatively clean water from industrial waste, while reducingthe volume of undesirable waste requiring disposal or further treatment.

Reverse osmosis requires relatively high pressures to be exerted on thehigh solute concentration side of the membrane. For instance, todesalinate seawater by conventional reverse osmosis techniques,pressures as high as 82 barg are commonly used to increase the recoveryof product water. This places a significant energy burden ondesalination methods that rely on conventional reverse osmosis.Moreover, streams having higher solute concentrations than seawater mayrequire even higher hydraulic pressures to be applied. Many commerciallyavailable reverse osmosis membranes are unsuitable for withstandinghydraulic pressures of greater than 82 barg. Accordingly, this canimpose a limitation on the concentration of feed solutions that can betreated using commercially available reverse osmosis membrane, whicheffectively limits the maximum concentration of the concentrated feedstream to an osmotic pressure equivalent to the maximum hydraulicpressure rating of the reverse osmosis membrane and pressure vessel.

DESCRIPTION

According to the present invention, there is provided a process forseparating solvent from a feed solution, said process comprising:

-   -   contacting a feed solution comprising solute dissolved in a        solvent with one side of a nanofiltration membrane,    -   applying hydraulic pressure to the feed solution, such that        solvent and some of the dissolved solute from the feed solution        flow through the nanofiltration membrane to provide a permeate        solution on the permeate-side of the nanofiltration membrane and        a concentrated solution on the retentate-side of the        nanofiltration membrane;    -   contacting the permeate solution from the nanofiltration        membrane with one side of a reverse osmosis membrane and        applying hydraulic pressure to the permeate solution, such that        solvent from the permeate solution flows through the reverse        osmosis membrane to leave a concentrated solution on the        retentate-side of the reverse osmosis membrane,    -   using the concentrated solution from the retentate-side of the        reverse osmosis membrane as at least part of the feed solution        to the nanofiltration membrane; and    -   withdrawing at least a portion of the concentrated solution from        the retentate-side of the nanofiltration membrane.

The withdrawn portion of concentrated solution from the retentate-sideof the nanofiltration membrane may be disposed of either directly orindirectly. In one example, the withdrawn portion may be combined withanother solution prior to disposal. In one embodiment, the withdrawnportion may be further concentrated prior to disposal. Alternatively,the withdrawn portion may be contacted with one side of a forwardosmosis membrane to draw water from source water on the opposite side ofthe forward osmosis membrane by forward osmosis. Where forward osmosisis defined as any osmotically driven membrane process, such as pressureenhanced osmosis, pressure assisted osmosis, osmosis and pressureretarded osmosis.

In the present invention, a feed solution comprising salts dissolved ina solvent is contacted with one side of a nanofiltration membrane. Thefeed solution may be saline ground water or surface water, brine,seawater or a waste stream. A nanofiltration membrane is selectedbecause it is a relatively “loose” membrane with a relatively highsolute permeability. Accordingly, as well as solvent, significantamounts of solute (e.g. salts) from the feed solution pass across thenanofiltration membrane to provide a permeate with a relatively highsolute concentration. Because the permeate has a relatively high soluteconcentration, the hydraulic pressure required to maintain the desiredlevel of flux across the nanofiltration membrane is relatively lowcompared, for example, to the hydraulic pressure required with, forexample, conventional reverse osmosis membranes having a lower solutepermeability.

In some embodiments, the feed solution is a waste stream. The feedsolution may contain multivalent cations and/or multivalent anions.Examples of multivalent cations include divalent cations and trivalentcations. Examples of divalent cations include alkaline earth metalcations, for instance, calcium, magnesium, strontium and barium.Examples of trivalent cations include aluminium. Examples of divalentanions include sulphate and carbonate anions.

The feed solution may have a high initial concentration of multivalentcations relative to the concentration of monovalent cations in the feed.In one example, the initial concentration of multivalent cations ishigher than the concentration of monovalent cations in the feed. In someembodiments, the initial concentration of multivalent cations may be atleast 20%, for example, at least 30% of the total cation concentrationin the feed. In one example, the initial concentration of multivalentcations may be at 20 to 100%, for instance, 20 to 90% of the totalcation concentration in the feed.

The feed solution may have a high initial concentration of multivalentanions relative to the concentration of monovalent anions in the feed.In one example, the initial concentration of multivalent anions ishigher than the concentration of monovalent anions in the feed. In someembodiments, the initial concentration of multivalent anions may be atleast 20%, for example, at least 30% of the total anion concentration inthe feed. In one example, the initial concentration of multivalent anionmay be at 20 to 100%, for instance, 20 to 90% of the total anionconcentration in the feed.

The feed solution may have a total concentration of multivalent cationsand multivalent anions that is high relative to the total concentrationof monovalent cations and monovalent anions in the feed. In one example,the initial concentration of multivalent cations and anions is higherthan the concentration of monovalent cations and anions in the feed. Insome embodiments, the initial concentration of multivalent cations andanions may be at least 20%, for example, at least 30% of the totalcation and anion concentration in the feed. In one example, the initialconcentration of multivalent cations may be at 20 to 100%, for instance,20 to 90% of the total cation and anion concentration in the feed.

Preferably, the process of the present invention comprises the step ofadding monovalent cation and/or monovalent anion to the feed solutionbefore the feed solution is contacted with the nanofiltration membrane.The monovalent cation and/or monovalent anion may be added in the formof solid salt (e.g. sodium chloride) or as a salt solution (e.g. sodiumchloride solution). The monovalent cation and/or monovalent anion may beadded to ensure that, when the resultant feed is passed through thenanofiltration membrane, the osmotic pressure of the permeate solutionon the permeate-side of the nanofiltration membrane is at least 50% ofthe osmotic pressure of the feed solution.

Nanofiltration membranes typically have relatively high monovalentsolute permeability relative to multivalent (e.g. divalent andtrivalent) solute permeability. Therefore, the nanofiltration membranemay limit the passage of multivalent solutes from the feed solutionthrough the membrane, thus limiting the solute concentration and osmoticpressure of the permeate from the nanofiltration membrane. In such casesa monovalent solute solution (for example sodium chloride) could bedosed to the feed solution. Because a nanofiltration membrane has arelatively high permeability to monovalent solutes, the soluteconcentration and osmotic pressure of the permeate from thenanofiltration membrane would be increased by the addition of themonovalent solutes. The permeate from the nanofiltration membrane iscontacted with one side of a reverse osmosis membrane and the monovalentsolutes in the nanofiltration membrane permeate would be retained on theretentate side of the reverse osmosis membrane and re-introduced to thenanofiltration feed.

The permeate from the nanofiltration membrane is contacted with one sideof a reverse osmosis membrane. Hydraulic pressure can be applied, suchthat solvent from the nanofiltration permeate flows through the reverseosmosis membrane to leave a concentrated solution on the retentate-sideof the reverse osmosis membrane. The permeate solution from the reverseosmosis membrane may be a product stream (e.g. product water) having areduced solute concentration. This product stream may optionally befurther treated, for example, to produce potable water or water forhousehold use. The water may also be used for example, for industrial ordomestic use.

The concentrated solution from the retentate-side of the reverse osmosismembrane is then used as at least part of the feed solution to thenanofiltration membrane. By using this concentrated solution as at leastpart of the feed, the concentration of the solution on theretentate-side of the nanofiltration membrane may be increased, allowinga highly concentrated solution to be withdrawn from the retentate-sideof the nanofiltration membrane. As mentioned above, the solutionwithdrawn from the retentate-side of the nanofiltration membrane may bedisposed of or further concentrated prior to disposal. Because of itshigh concentration, the volume of liquid requiring disposal or treatmentis reduced. Thus, where the withdrawn stream is intended for treatmentin a downstream evaporator or crystalliser, the capacity and/or heatdemand of such equipment may be reduced. Because of its highconcentration, the solution from the retentate-side of thenanofiltration membrane may also be withdrawn and used as a drawsolution in a osmotically driven membrane process.

Counter-intuitively, the present invention employs a loose or highsolute passage (i.e. nanofiltration) membrane to provide highlyconcentrated solutions on the retentate-side of the membrane.Specifically, the present invention employs a nanofiltration membrane intandem with a reverse osmosis membrane to generate a product stream(e.g. product water) having a reduced solute concentration as well as ahighly concentrated solution e.g. for ease of disposal/further treatmentor use as a draw solution for a direct osmosis process. By using ananofiltration membrane, the concentration of the highly concentratedretentate solution capable of being produced by the process of thepresent invention is greater than could be produced using reverseosmosis alone operating under the same hydraulic pressure limitations.Furthermore, by re-circulating the retentate from the reverse osmosismembrane as at least part of the feed to the nanofiltration membrane,highly concentrated waste streams can be produced, reducing the volumeof waste requiring disposal or further treatment.

The benefits of embodiments of the invention are that:

-   -   (a) a higher concentration brine stream (and therefore lower        volume) can be produced than by using RO alone    -   (b) higher concentration feed water can be desalinated than        using RO alone and/or    -   (c) a higher recovery of product water can be achieved than RO        treating the same concentration of feed water.

All of the above benefits can be achieved with no increase to the normaloperational pressures employed when RO alone is used.

When withdrawn, the withdrawn portion of concentrated solution form theretentate-side of the nanofiltration membrane may have a total dissolvedsalts concentration of at least 90,000 mg/l, preferably at least 95,000mg/l. In one embodiment, the withdrawn portion of concentrated solutionform the retentate-side of the nanofiltration membrane may have a totaldissolved salts concentration of at least 100,000 mg/l, for example, atleast 120,000 mg/l. In one example, the withdrawn portion has a totaldissolved salts concentration of at least 130,000 mg/l.

When withdrawn, the withdrawn portion of concentrated solution form theretentate-side of the nanofiltration membrane may have an osmoticpressure of at least 75 barg, preferably at least 80 barg. In oneembodiment, the withdrawn portion of concentrated solution form theretentate-side of the nanofiltration membrane may have an osmoticpressure of at least 110 barg, for instance, at least 120 barg.

Preferably, the withdrawn portion of concentrated solution from theretentate-side of the nanofiltration membrane is further concentratedusing a thermal evaporator or crystalliser.

In one embodiment, the concentrated solution from the retentate-side ofthe reverse osmosis membrane is combined with a further solute solutionand the combined stream used as the feed solution to the nanofiltrationmembrane. The further solute solution may be, for example, saline groundwater or surface water, brine, seawater, or a waste stream.

In one embodiment, the permeate solution from the nanofiltrationmembrane is combined with a further solute solution and the combinedstream contacted with one side of the reverse osmosis membrane. Thefurther solute solution may be, for example, saline ground water orsurface water, brine, seawater or a waste stream.

In one embodiment, the withdrawn portion of the concentrated solutionfrom the retentate-side of the nanofiltration membrane is concentratedby contacting said withdrawn portion with one side of a furthersemi-permeable membrane. The semi-permeable membrane may be as permeable(e.g. comparable average pore size) or less permeable (e.g. smalleraverage pore size) than the nanofiltration membrane. For example, thesemi-permeable membrane may be a nanofiltration membrane or a reverseosmosis membrane. Hydraulic pressure may be applied to the withdrawnportion, such that solvent from said portion flows through the furthersemi-permeable membrane to provide a permeate solution on thepermeate-side of the further semi-permeable membrane and a retentatesolution on the retentate-side of the further semi-permeable membrane.The retentate solution on the retentate-side of the furthersemi-permeable membrane may be withdrawn and disposed of, concentratedfurther e.g. prior to disposal or contacted with one side of a directosmosis membrane to draw water from a source solution on the oppositeside of the direct osmosis membrane by direct osmosis i.e. in anosmotically driven membrane process. The permeate solution frompermeate-side of the further semi-permeable membrane may be combinedwith the concentrated solution from the retentate-side of the reverseosmosis membrane and introduced into the nanofiltration membrane.Preferably, the further semi-permeable membrane is a nanofiltrationmembrane.

In one embodiment, prior to being used as at least a portion of the feedsolution to the nanofiltration membrane, the concentrated solution fromthe retentate-side of the reverse osmosis membrane is passed through anadditional semi-permeable membrane to provide a permeate solution on thepermeate-side of the additional semi-permeable membrane and a retentatesolution on the retentate-side of the additional semi-permeablemembrane. The permeate solution may be used as at least a portion of thefeed to the nanofiltration membrane. The retentate solution on theretentate-side of the additional semi-permeable membrane may bewithdrawn and disposed of, further concentrated or contacted with adirect osmosis membrane to draw water from a source solution on theopposite side of the direct osmosis membrane by direct osmosis i.e. inan osmotically driven membrane process.

The withdrawn portion of concentrated solution on the retentate-side ofthe nanofiltration membrane may be concentrated by passing the withdrawnportion through the additional (or yet another) semi-permeable membraneand applying hydraulic pressure to said withdrawn portion such thatsolvent from said portion flows through the membrane to provide apermeate solution on the permeate-side of the semi-permeable membraneand a retentate solution on the retentate-side of the membrane. Thisretentate may be withdrawn and then optionally disposed of, concentratedfurther e.g. prior to disposal or contacted with a direct osmosismembrane to draw water from a source solution on the opposite side ofthe direct osmosis membrane by direct osmosis i.e. in an osmoticallydriven membrane process. Preferably, the withdrawn portion ofconcentrated solution on the retentate-side of the nanofiltrationmembrane is combined with the concentrated solution from theretentate-side of the reverse osmosis membrane and the combined streamis passed through the additional semi-permeable membrane.

The additional or yet another semi-permeable membrane as permeable as orless permeable than the nanofiltration membrane. For example, theadditional or yet another semi-permeable membrane may be ananofiltration membrane or a reverse osmosis membrane. Preferably, theadditional semi-permeable membrane may be a nanofiltration membrane.Where employed, the yet another semi-permeable membrane may also be ananofiltration membrane.

Where the withdrawn portion of concentrated solution on theretentate-side of the nanofiltration membrane is contacted with afurther membrane, the further membrane may have an average pore size orpermeability that is no more than 100 times greater, preferably no morethan 50 times greater, more preferably no more than 10 times greaterthan the average pore size or permeability of the nanofiltrationmembrane. The withdrawn portion of concentrated solution on theretentate-side of the nanofiltration membrane may be contacted with afurther membrane that has an average pore size or permeability that isless than 10 times greater, for example, less than 5 times greater thanthe average pore size or permeability of the nanofiltration membrane.For example, the further membrane may have substantially the same or alower average pore size or permeability as the nanofiltration membrane.As noted above, the further membrane may be a nanofiltration membrane orreverse osmosis membrane. The further membrane is preferably notselected from a particle filtration membrane, a microfiltration membraneor an ultrafiltration membrane. The further membrane may have an averagepore size that is less than 0.1 microns, for example, less than 0.05microns.

The feed solution may be any solution, such as an aqueous solution. Thefeed solution may be a salt solution, for example, an aqueous saltsolution. In some embodiments, the feed solution is an aqueous solutionof sodium chloride. Examples of suitable feed solutions include salineground water or surface water, brine and seawater. Other examplesinclude waste water streams, lake water, river water and pond water.Examples of waste water streams include industrial or agricultural wastewater streams.

The total dissolved salt concentration of the feed solution to thenanofiltration membrane may be at least 5,000 mg/l, for example, 5,000to 140,000 mg/l. In one example, the total dissolved salt concentrationof the feed solution to the nanofiltration membrane is at least 30,000mg/l. The osmotic pressure of the feed may be at least 4 barg, forexample, 4 to 130 barg.

The nanofiltration membrane may be selected such that sufficientdissolved salt passes through the nanofiltration membrane, whereby thetotal dissolved salts concentration or osmotic pressure of the permeatesolution on the permeate-side of the nanofiltration membrane is at least30%, for example, at least 50% or at least 70% of the osmotic pressureof the solution fed to the nanofiltration membrane. For example, theosmotic pressure of the permeate solution on the permeate-side of thenanofiltration membrane is 50 to 90% of the osmotic pressure of thesolution fed to the nanofiltration membrane.

When withdrawn, the withdrawn portion of concentrated solution form theretentate-side of the nanofiltration membrane may have a total dissolvedsalts concentration that is at least 1.1 times, for example, at least 2or 3 times the total dissolved salt concentration of the feed.

When withdrawn, the withdrawn portion of concentrated solution form theretentate-side of the nanofiltration membrane may have an osmoticpressure that is at least 1.1 times, for example, at least 2 or 3 timesthe osmotic pressure of the feed.

The membrane employed in the nanofiltration step may have an average(e.g. mean) pore size of 4 to 80 Angstroms. Preferably, the average(e.g. mean) pore size of the membrane is 20 to 70 Angstroms, morepreferably 30 to 60 Angstroms, and most preferably 40 to 50 Angstroms.Pore size (e.g. mean pore size) may be measured using any suitabletechnique. For example, a differential flow method may be employed(Japan Membrane Journal, vol. 29; no. 4; pp. 227 -235 (2004)) or the useof salts, uncharged solutes and atomic force microscopy (Journal ofMembrane Science 126 (1997) 91-105).

The membranes used in the nanofiltration step may be cast as a “skinlayer” on top of a support formed, for example, of a microporous polymersheet. The resulting membrane may have a composite structure (e.g. athin-film composite structure).

Typically, the separation properties of the membrane are controlled bythe pore size and electrical charge of the “skin layer”.

Examples of suitable nanofiltration membranes include ESNA-1(Hydranautics, Oceanside, Calif.), SR 90, NF-270, NF 90, NF 70, NF 50,NF 40, NF 40 HF membranes (Dow FilmTech, Minneapolis, Minn.), TR-60, SU600 membrane (Toray, Japan) and NRT 7450 and NTR 7250 membranes (NittoElectric, Japan).

The nanofiltration membrane may be planar or take the form of a tube orhollow fibre. For example, a tubular configuration of hollow fine fibremembranes may be used. If desired, the membrane may be supported on asupporting structure, such as a mesh support. When a planar membrane isemployed, the sheet may be rolled such that it defines a spiral incross-section. When a tubular membrane is employed, one or more tubularmembranes may be contained within a housing or shell. The solution maybe introduced into the housing, whilst the solvent may be removed as afiltrate from the tubes or vice-versa.

The nanofiltration step may also be carried out at an elevated pressure.For example, the nanofiltration step may be carried out at a pressure of25 to 120 bar, preferably 40 to 100 bar, more preferably 50 to 80 bar.As mentioned above, solution from the retentate-side of the selectivemembrane of the reverse osmosis step is passed through thenanofiltration membrane. Since this solution is on the high pressureside of the membrane, it may not be necessary to apply further pressureto the solution as it passes through the nanofiltration membrane.However, it is possible to apply further pressure to the solution as itpasses through the nanofiltration membrane, if desired.

Any suitable reverse osmosis membrane may be used in the presentinvention. For example, the reverse osmosis membrane may have an average(e.g. mean) pore size of 0.5 to 80 Angstroms, preferably, 2 to 50Angstroms. In a preferred embodiment, the membrane has an average (e.g.mean) pore size of from 3 to 30 Angstroms. Pore size (e.g. mean poresize) may be measured using any suitable technique. For example, adifferential flow method may be employed (Japan Membrane Journal, vol.29; no. 4; pp. 227 -235 (2004)) or the use of salts, uncharged solutesand atomic force microscopy (Journal of Membrane Science 126 (1997)91-105).

Suitable reverse osmosis membranes include integral membranes andcomposite membranes. Specific examples of suitable membranes includemembranes formed of cellulose acetate (CA) and/or cellulose triacetate(CTA), such as or similar to those used in the study of McCutcheon etal., Desalination 174 (2005) 1-11 and membranes formed of polyamide(PA). An array of membranes may be employed.

The reverse osmosis membrane may be planar or take the form of a tube orhollow fibre. For example, a tubular configuration of hollow fine fibremembranes may be used. If desired, the membrane may be supported on asupporting structure, such as a mesh support. When a planar membrane isemployed, the sheet may be rolled such that it defines a spiral incross-section. When a tubular membrane is employed, one or more tubularmembranes may be contained within a housing or shell.

The reverse osmosis membrane may be carried out at an elevated pressureto drive the (liquid) solution through the membrane. For example, thereverse osmosis step may be carried out at a pressure of 25 to 120 bar,preferably 50 to 100 bar, more preferably 60 to 80 bar.

These and other aspects of the present invention will now be describedwith reference to the accompanying drawings in which:

FIG. 1 is a schematic drawing of a system for performing a firstembodiment of the process of the present invention;

FIG. 2 is a schematic drawing of a system for performing a secondembodiment of the process of the present invention;

FIG. 3 is a schematic drawing of a system for performing a thirdembodiment of the process of the present invention; and

FIG. 4 is a schematic drawing of a system for performing a fourthembodiment of the process of the present invention.

Referring to FIG. 1, this drawing depicts a system comprising ananofiltration membrane unit 10 comprising a nanofiltration membrane 10a and a reverse osmosis membrane unit 12 comprising a reverse osmosismembrane 12 a. In use, a feed solution (e.g. wastewater) comprisingsolutes dissolved in a solvent is contacted with one side of thenanofiltration membrane 10 a. Hydraulic pressure is applied to the feedsolution, such that solvent (water) and some of the dissolved salts fromthe feed solution flow through the nanofiltration membrane to provide apermeate solution 14 on the permeate-side of the nanofiltration membrane10 a and a concentrated solution 16 on the retentate-side of thenanofiltration membrane.

The permeate solution 14 from the nanofiltration membrane 10 a iswithdrawn via conduit 18 and contacted with one side of the reverseosmosis membrane 12 a. Hydraulic pressure is applied to the solution,such that solvent from the solution flows through the reverse osmosismembrane 12 a to leave a concentrated solution 20 on the retentate-sideof the reverse osmosis membrane 12 a and a product solution 22 on thepermeate side of the reverse osmosis membrane 12 a. The product solution22 advantageously has a relatively low solute (e.g. salt) concentration.

The concentrated solution 20 from the retentate-side of the reverseosmosis membrane 12 a is withdrawn via conduit 26 and used as at leastpart of the feed solution to the nanofiltration membrane 10 a. In thisembodiment, it can be combined with fresh feed (e.g. wastewater) inconduit 8 and the combined stream may be fed to the nanofiltration unit10.

At least a portion of the concentrated solution 16 from theretentate-side of the nanofiltration membrane 10 a is withdrawn viaconduit 24. This solution 24 may be disposed of or further concentrated,for example, using thermal methods (not shown). As the solution 24 ishighly concentrated, the volume of concentrated waste requiringtreatment/disposal is relatively small as compared, for instance, to thevolume of concentrated waste that would be produced using reverseosmosis (RO) alone.

The feed (e.g. wastewater) may contain divalent cations and/or anions,for example, calcium, magnesium, strontium and/or barium cations, and/orsulphate and/or carbonate anions. The initial concentration of divalentcations and anions is higher than the concentration of monovalentcations and anions in the feed. For example, the initial concentrationof divalent cations or anions may be at 20 to 90% of the total cationand anion concentration in the feed.

Monovalent cations and monovalent anions may be added to the feedsolution via line 50 before the feed solution is contacted with thenanofiltration membrane. The monovalent cation and/or monovalent anionmay be added in the form of solid salt (e.g. sodium chloride) or as asalt solution (e.g. sodium chloride solution). The monovalent cationand/or monovalent anion may be added to ensure that, when the resultantfeed is passed through the nanofiltration membrane, the osmotic pressureof the permeate solution on the permeate-side of the nanofiltrationmembrane is at least 50% of the osmotic pressure of the feed solution.

Nanofiltration membranes typically have relatively high monovalentsolute permeability relative to multivalent (e.g. divalent andtrivalent) solute permeability. Therefore, the nanofiltration membranemay limit the passage of multivalent solutes from the feed solutionthrough the membrane, thus limiting the solute concentration and osmoticpressure of the permeate from the nanofiltration membrane. In such casesa monovalent solute solution (for example sodium chloride) could bedosed to the feed solution. Because a nanofiltration membrane has arelatively high permeability to monovalent solutes, the soluteconcentration and osmotic pressure of the permeate from thenanofiltration membrane would be increased by the addition of themonovalent solutes. The permeate from the nanofiltration membrane iscontacted with one side of a reverse osmosis membrane and the monovalentsolutes in the nanofiltration membrane permeate would be retained on theretentate side of the reverse osmosis membrane and re-introduced to thenanofiltration feed.

FIG. 2 depicts a system for performing an alternative embodiment of theprocess described with reference to FIG. 1. Like parts have beenlabelled with like reference numerals. Like in FIG. 1, a monovalentcations and/or anions (e.g. sodium chloride) may be added to the feedvia line 50. However, in this embodiment, the concentrated solution 20from the retentate-side of the reverse osmosis membrane 12 a iswithdrawn via conduit 26 and used wholly as the feed solution to thenanofiltration membrane 10 a. Unlike the embodiment depicted in FIG. 1,the concentrated solution 20 is not combined with fresh feed (e.g.wastewater). However, the feed to the reverse osmosis unit 12 is onlyformed in part by the permeate (see conduit 18) from the nanofiltrationmembrane 10 a. This permeate is combined with fresh feed (e.g.wastewater) from conduit 8 and the combined feed is introduced into thereverse osmosis unit 12.

FIG. 3 depicts a system for performing a third embodiment of the processof the present invention. The system is similar to that described inFIG. 1 and like parts have been labelled with like numerals. Like inFIG. 1, a monovalent cations and/or anions (e.g. sodium chloride) may beadded to the feed via line 50. However, in this embodiment, thewithdrawn portion (see conduit 24) of the concentrated solution 16 fromthe retentate-side of the nanofiltration membrane 10 a is concentratedby contacting the withdrawn portion with one side of a furthersemi-permeable membrane (e.g. a further nanofiltration membrane) 28. Thewithdrawn portion (see conduit 24) may optionally be combined with freshfeed prior to contact with the further semi-permeable membrane 28.Hydraulic pressure is then applied, such that water from the withdrawnportion flows through the further semi-permeable membrane 28 (e.g. afurther nanofiltration membrane) to provide a permeate solution 30 onthe permeate-side of the further semi-permeable membrane 28 and aretentate solution on the retentate-side of the further semi-permeablemembrane. The retentate solution on the retentate-side of the furthersemi-permeable membrane 28 is withdrawn via conduit 32 and disposed ofor concentrated further prior to disposal. The permeate 30 is withdrawnvia conduit 34 where it is combined with the concentrated solution fromthe reverse osmosis unit 12 in conduit 26 and introduced into thenanofiltration membrane unit 10.

FIG. 4 depicts a system for performing a fourth embodiment of theprocess of the present invention. The system is similar to thatdescribed in FIG. 1 and like parts have been labelled with likenumerals. Like in FIG. 1, a monovalent cations and/or anions (e.g.sodium chloride) may be added to the feed via line 50. However, prior tobeing used as at least a portion of the feed to the nanofiltrationmembrane 10 a, the concentrated solution 20 from the retentate-side ofthe reverse osmosis membrane 12 a is passed through an additionalsemi-permeable membrane 40 (e.g. an additional nanofiltration membrane)to provide a permeate solution 42 on the permeate-side of the additionalsemi-permeable membrane 40 and a retentate solution 44 on theretentate-side of the additional semi-permeable membrane 40. Thepermeate solution 42 is used as the feed to the nanofiltration membrane10 a. In this embodiment, rather than being e.g. untreated wastewater46, the feed to the nanofiltration unit 10 is wastewater 46 that hasbeen pre-treated in the reverse osmosis unit 12 and by the additionalsemi-permeable membrane 40.

The retentate solution on the retentate-side of the additionalsemi-permeable membrane is withdrawn via conduit 48 and disposed of orfurther concentrated.

In this embodiment, the withdrawn portion of concentrated solution 16 onthe retentate-side of the nanofiltration membrane 10 a is withdrawn viaconduit 24 and is concentrated by passing said withdrawn portion throughthe additional semi-permeable membrane 40. The feed to the additionalsemi-permeable membrane 40, therefore, consists of the concentratedsolution 20 from the retentate-side of the reverse osmosis membrane 12 aas well as the concentrated solution 16 that is withdrawn from thenanofiltration unit 10 via conduit 24. When hydraulic pressure isapplied, a permeate solution 42 is provided on the permeate-side of thesemi-permeable membrane 40 and a retentate solution on theretentate-side of the membrane. The retentate is withdrawn via conduit48 as described above.

The additional semi-permeable membrane 40 may be a nanofiltrationmembrane.

EXAMPLES Example 1

In this modelled Example, a waste water stream having a total dissolvedsalts (TDS) concentration of 43218 g/l and an osmotic pressure of 34barg was treated using the embodiment of the invention shownschematically in FIG. 1. FIG. 5 shows the points at which the flowstreams were sampled and analysed. Table 1 below shows the TDS,pressures, osmotic pressures and flow rates of the various streams. Ascan be seen from Table 1, 68% of the water in the waste water stream wasrecovered using this embodiment of the invention (N.B. systemrecovery=(flow of product water)/(flow of feed water) or the percentageof the feed water that is converted to product water.

TABLE 1 STREAM 1 2 3 4 5 6 TDS (mg/l) 43218 61453 45267 86269 133503 868Pressure (barg) 0 50 77 75 46 0 Osmotic Pressure 34 50 36 73 119 1(barg) Flow (m³/hr) 100 174 142 74 32 68 System Recovery 68 (%)

Comparative Example 2

In this modelled Comparative Example, the waste water stream treated inExample 1 was treated using the process shown schematically in FIG. 6.FIG. 6 depicts a process that is similar to that shown in FIGS. 1 and 5,except that the concentrated solution from the retentate-side of thereverse osmosis membrane is not used as at least part of the feedsolution to the nanofiltration membrane. Instead, this concentratedsolution is combined with the concentrated solution from theretentate-side of the nanofiltration membrane and withdrawn fordisposal. Table 2 below shows the TDS, pressures, osmotic pressures andflow rates of the various streams. As can be seen from Table 2, only 56%of the water in the waste water stream was recovered using the processdepicted in FIG. 6. Furthermore, the TDS of the concentrated wastestream is less than the maximum TDS achieved in Example 1.

TABLE 2 STREAM 1 2 3 4 5 6 TDS (mg/l) 43218 33634 129119 89012 98257 750Pressure (barg) 50 77 50 76 50 0 Osmotic Pressure 34 27 109 76 83 1(barg) Flow (m³/hr) 100 90 10 34 44 56 System Recovery 56 (%)

Comparative Example 3

In this modelled Comparative Example, the waste water stream treated inExample 1 was treated using the process shown schematically in FIG. 7.FIG. 7 depicts a standard reverse osmosis process in which the feedwaste water stream is contacted with a reverse osmosis membrane.Hydraulic pressure is applied to produce product water on the permeateside of the reverse osmosis membrane. The concentrated solution on theretentate side of the reverse osmosis membrane is withdrawn. Table 3below shows the TDS, pressures, osmotic pressures and flow rates of thevarious streams. As can be seen from Table 3, only 49% of the water inthe waste water stream was recovered using the process of FIG. 7.Furthermore, the TDS of the concentrated waste stream is less than themaximum TDS achieved in Example 1.

The Examples above were modelled using DOW membrane software package(ROSA) and a simple mass balance to determine stream data not provideddirectly by the DOW projections. All projections were run at 30° C.

TABLE 3 STREAM 1 2 3 TDS (mg/l) 43218 84178 722 Pressure (barg) 77 74 0Osmotic Pressure (barg) 34 70 1 Flow (m³/hr) 100 51 49 System Recovery(%) 49

1. A process for separating solvent from a feed solution, said processcomprising: contacting a feed solution comprising solutes dissolved in asolvent with one side of a nanofiltration membrane, applying hydraulicpressure to the feed solution, such that solvent and some of thedissolved salts from the feed solution flow through the nanofiltrationmembrane to provide a permeate solution on the permeate-side of thenanofiltration membrane and a concentrated solution on theretentate-side of the nanofiltration membrane; contacting the permeatesolution from the nanofiltration membrane with one side of a reverseosmosis membrane and applying hydraulic pressure to the permeatesolution, such that solvent from the permeate solution flows through thereverse osmosis membrane to leave a concentrated solution on theretentate-side of the reverse osmosis membrane, using the concentratedsolution from the retentate-side of the reverse osmosis membrane as atleast part of the feed solution to the nanofiltration membrane;withdrawing at least a portion of the concentrated solution from theretentate-side of the nanofiltration membrane.
 2. The process as claimedin claim 1, whereby, if the withdrawn portion of the concentratedsolution from the retentate-side of the nanofiltration membrane iscontacted with a further membrane, the further membrane has an averagepore size or permeability that is no more than 100 times greater thanthe average pore size or permeability of the nanofiltration membrane. 3.The process as claimed in claim 1, wherein the withdrawn portion of theconcentrated solution from the retentate-side of the nanofiltrationmembrane is i) disposed of; ii) further concentrated prior to disposal;or iii) contacted with one side of a direct osmosis membrane to drawwater from source water on the opposite side of the direct osmosismembrane by direct osmosis.
 4. The process as claimed in claim 3,wherein the withdrawn portion of concentrated solution from theretentate-side of the nanofiltration membrane is concentrated using amembrane, thermal evaporator or crystalliser.
 5. The process as claimedin claim 1, wherein the nanofiltration membrane is selected such thatsufficient dissolved solute passes through the nanofiltration membrane,whereby the osmotic pressure of the permeate solution on thepermeate-side of the nanofiltration membrane is at least 50% of theosmotic pressure of the feed solution.
 6. The process as claimed inclaim 1, wherein the concentrated solution from the retentate-side ofthe reverse osmosis membrane is combined with a further salt solutionand the combined stream used as the feed solution to the nanofiltrationmembrane.
 7. The process as claimed in claim 1, wherein the permeatesolution from the nanofiltration membrane is combined with a furthersalt solution and the combined stream contacted with one side of thereverse osmosis membrane.
 8. The process as claimed in claim 3, whereinthe withdrawn portion of the concentrated solution from theretentate-side of the nanofiltration membrane is concentrated bycontacting said withdrawn portion with one side of a furthersemi-permeable membrane, and applying hydraulic pressure to saidwithdrawn portion, such that solvent from said portion flows through thefurther semi-permeable membrane to provide a permeate solution on thepermeate-side of the further semi-permeable membrane and a retentatesolution on the retentate-side of the further semi-permeable membrane,wherein the retentate solution on the retentate-side of the furthersemi-permeable membrane is withdrawn and disposed of or concentratedfurther prior to disposal.
 9. The process as claimed in claim 8, whereinthe permeate solution from permeate-side of the further semi-permeablemembrane is combined with the concentrated solution from theretentate-side of the reverse osmosis membrane and introduced into thenanofiltration membrane.
 10. The process as claimed in claim 8, whereinthe further semi-permeable membrane is a nanofiltration membrane. 11.The process as claimed in claim 1, wherein, prior to being used as atleast a portion of the feed solution to the nanofiltration membrane, theconcentrated solution from the retentate-side of the reverse osmosismembrane is passed through an additional semi-permeable membrane toprovide a permeate solution on the permeate-side of the additionalsemi-permeable membrane and a retentate solution on the retentate-sideof the additional semi-permeable membrane, which permeate solution isused as at least a portion of the feed to the nanofiltration membrane.12. The process as claimed in claim 11, wherein the retentate solutionon the retentate-side of the additional semi-permeable membrane iswithdrawn and disposed of or further concentrated.
 13. The process asclaimed in claim 11, wherein the withdrawn portion of concentratedsolution on the retentate-side of the nanofiltration membrane isconcentrated by passing said withdrawn portion through the additionalsemi permeable membrane and applying hydraulic pressure to saidwithdrawn portion such that solvent from said portion flows through themembrane to provide a permeate solution on the permeate-side of thesemi-permeable membrane and a retentate solution on the retentate-sideof the membrane, which retentate is withdrawn and disposed of orconcentrated further prior to disposal.
 14. The process as claimed inclaim 13, wherein the withdrawn portion of concentrated solution on theretentate-side of the nanofiltration membrane is combined with theconcentrated solution from the retentate-side of the reverse osmosismembrane and the combined stream is passed through the additionalsemi-permeable membrane.
 15. The process as claimed in claim 11, whereinthe additional semi-permeable membrane is a nanofiltration membrane. 16.The process as claimed in claim 1, wherein the solution that permeatesthe reverse osmosis membrane is withdrawn as product water.
 17. Theprocess as claimed in claim 1, wherein, when withdrawn, the withdrawnportion of concentrated solution form the retentate-side of thenanofiltration membrane may have a total dissolved salts concentrationof at least 90,000 mg/l.
 18. The process as claimed in claim 1, whereinthe feed solution has an initial concentration of multivalent cationsand multivalent anions that is greater than the concentration ofmonovalent cations and monovalent anions, and wherein the processfurther comprises the step of adding monovalent cation and/or monovalentanion to the feed solution before the feed solution is contacted withthe nanofiltration membrane.
 19. The process as claimed in claim 18,wherein the monovalent cation and/or monovalent anion is added to thefeed solution to raise the osmotic pressure of the permeate solution onthe permeate-side of the nanofiltration membrane to at least 50% of theosmotic pressure of the feed solution.
 20. The process as claimed inclaim 18, wherein the withdrawn portion of the concentrated solutionfrom the retentate-side of the nanofiltration membrane is concentratedby contacting said withdrawn portion with one side of a furthernanofiltration membrane, and applying hydraulic pressure to saidwithdrawn portion, such that solvent from said portion flows through thefurther nanofiltration membrane to provide a permeate solution on thepermeate-side of the further nanofiltration membrane and a retentatesolution on the retentate-side of the further nanofiltration membrane,wherein the retentate solution on the retentate-side of the furthernanofiltration membrane is withdrawn and disposed of or concentratedfurther prior to disposal.
 21. The process as claimed in claim 20,wherein, prior to contact with the further nanofiltration membrane,monovalent cation and/or monovalent anion are added to the withdrawnportion.